Modified RNA Ligase for Efficient 3` Modification of RNA

ABSTRACT

The invention provides a novel truncated mutated T4 RNA ligase 2. In addition, methods are provided for ligating pre-adenlylated donor molecules to the 3′ hydroxyl group of RNA in the absence of ATP using the ligase.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/525,176, filed on Jul. 30, 2009, which is a U.S. National Phase ofInternational Application No. PCT/US2008/001227, filed Jan. 30, 2008,which claims priority from U.S. Provisional Application No. 60/887,288,filed Jan. 30, 2007, all of which are incorporated herein by reference.

The invention described in this application was made with financialsupport from the National Institutes of Health, Grant Number P01GM073047-01. The United States government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

RNA ligases are used for 3′-labeling of RNAs (the acceptor) byphosphorylated nucleotide analogs or oligonucleotides (the donor) in thepresence of ATP (Aravin, 2005; Pfeffer, 2004). The reaction generallyrequires ATP because the donor molecule 5′ phosphate (p) needs to beadenylated by the RNA ligase. The RNA ligase subsequently positions theacceptor molecule 3′ hydroxyl terminus for attack on the adenylateddonor phosphate (App) resulting in departure of the adenylate in theform of adenosine phosphate (AMP). The result is the formation of a 3′acceptor/5′ donor phosphodiester linkage.

The requirement for ATP in ligation is eliminated if pre-adenylatedcompounds are provided (England, 1977). Non-nucleotidic pre-adenylatedcompounds can also be used as donor molecule substrates. Biotin orfluorescent dyes have been ligated to the 3′ end of tRNAs in thismanner.

Most of the literature and commercial products use conventional T4 RNAligase 1 (Rnl1), but more recently a second ligase has been describedand characterized from phage T4, known as T4 RNA ligase 2 (Rnl2) (Ho andShuman 2002). T4 Rnl2 is a 334 amino acid residue ligase that, likeRnl1, catalyzes intramolecular and intermolecular RNA strand ligation.In contrast to Rnl1, Rnl2 shows nick-sealing activity in adouble-stranded RNA or an RNA-DNA context (Nandakumar et al. 2004). Atruncated form of this ligase comprising amino acids 1-249 has beenshown to maintain adenylyltransferase and AppRNA ligase activity.Deletion of amino acids 34 or 227 in full-length Rnl2 can inactivate theenzyme (Yin et al., 2003), indicating that N-terminal or C-terminaldeletions of the enzyme beyond these points very likely would abolishligase activity. Conservative mutation of residue K227 to Q rescues theactivity of ligating pre-adenylated donor RNAs to acceptor RNAs butcompromises the enzymes adenylate transfer activity. Some otherresidues, such as D120, K209, and K225 when mutated also differentiallyaffect ligation of the pre-adenylated donor versus adenylate transferactivity (Yin et al., 2003).

Rnl2(1-249) has, due to its missing C-terminal domain, a reducedaffinity for binding phosphate donors and therefore transfers theadenylate residue from the adenylated enzyme to the 5′-phosphate groupof miRNA acceptors less efficiently then other ligases. Therefore,Rnl2(1-249) allows consistently better labeling results than Rnl1.Nevertheless, the ratio of desired ligation versus unwanted sidereactions, such as circularization and dimerization, still depends onthe kinetic parameters of individual steps of the ligase mechanism.

Circularization is a consequence of deadenylation of pre-adenylateddonors followed by adenylate transfer to miRNA 5′ phosphates formingApp-miRNA that will then circularize by attack of the miRNA 3′ hydroxyland also dimerize to a certain degree.

Circularization can be partially suppressed by the use of highconcentration donors or reduction of temperature but in cannot beavoided completely. These side reactions are mostly unpredictable andcaused by sequence-dependent secondary structure variation of donor andacceptor molecules.

miRNAs are 21- to 23-nt RNA molecules that act as natural regulators ofgene expression in plants and animals. In humans about 400 miRNA geneshave been identified, and methods to characterize their tissue orcell-type specific expression patterns and their deregulation in diseaseare needed (Aravin, 2005). miRNAs are naturally 5′ phosphorylated andcarry 2′,3′ dihydroxyl termini.

One of the approaches for detecting miRNAs is based on microarrayhybridization that requires fluorescent labeling of the miRNA sample. AnRNA ligase is used to conjugate a fluorescently labeled donor to themiRNA. However, the current methods of ligation are plagued by theunwanted side reactions described above.

Accordingly, a need exists for an improved RNA ligase enzyme that canmore efficiently modify the 3′ position of RNA.

SUMMARY OF THE INVENTION

In a first aspect of the invention an enzyme is provided. The enzymeincludes a truncated T4 RNA ligase 2 lacking a C-terminal segmentstarting with amino acid residue 228 or any higher amino acid residue,and optionally lacking an N-terminal segment starting before amino acidresidue 34 or any lower amino acid residue, wherein said enzyme includesa substitution at a location selected from the group consisting oflysine at position 225, lysine at position 227, arginine at position 55,or a combination thereof, with a naturally occurring amino acid, andwherein the truncated T4 RNA ligase is capable of modifying a 3′hydroxyl group of RNA.

In another aspect of the invention, a method is provided forenzymatically ligating a pre-adenylated donor molecule to RNA. Themethod includes reacting the pre-adenylated donor molecule and the 3′hydroxyl group of a 5′ phosphorylated or de-phosphorylated RNA in theabsence of adenosine triphosphate and in the presence of an enzymecomprising a truncated T4 RNA ligase 2 lacking a C-terminal segmentstarting with amino acid residue 228 or any higher amino acid residue,and optionally lacking an N terminal segment before amino acid residue34 or any lower amino acid residue, wherein said enzyme includes asubstitution at a location selected from the group consisting of lysineat position 225, lysine at position 227, arginine at position 55, or acombination thereof, with a naturally occurring amino acid, and whereinthe truncated T4 RNA ligase 2 is capable of ligating the pre-adenylateddonor molecule to the 3′ hydroxyl group of the optionallyde-phosphorylated RNA in the absence of adenosine triphosphate.

In another aspect of the invention, a method is provided forenzymatically ligating a pre-adenylated donor molecule tode-phosphorylated RNA. The method includes reacting the pre-adenylateddonor molecule with the 3′ hydroxyl group of the RNA in the absence ofadenosine triphosphate, and in the presence of an enzyme comprising atruncated T4 RNA ligase 2 lacking a C-terminal segment starting withamino acid residue 228 or any higher amino acid residue, and optionallylacking an N terminal segment before amino acid residue 34 or any loweramino acid residue, wherein when both the donor molecule and/or theacceptor molecule are each adenylated at the 5′ position and have a freehydroxyl group at the 3′ position, the molecule has fewer than sixteennucleotide residues; and wherein the truncated T4 RNA ligase 2 iscapable of ligating the pre-adenylated donor molecule to the 3′ hydroxylgroup of the de-phosphorylated RNA in the absence of adenosinetriphosphate.

In another aspect of the invention, a nucleic acid molecule is providedthat encodes an enzyme that includes a truncated T4 RNA ligase 2 lackinga C-terminal segment starting with amino acid residue 228 or any higheramino acid residue, and optionally lacking an N-terminal segmentstarting before amino acid residue 34 or any lower amino acid residue,wherein said enzyme includes a substitution at a location selected fromthe group consisting of lysine at amino acid position 225, lysine atamino acid position 227, arginine at amino acid position 55, or acombination thereof, with a naturally occurring amino acid, and whereinthe truncated T4 RNA ligase is capable of modifying a 3′ hydroxyl groupof RNA.

In another aspect of the invention, a kit is provided. The kit includesan enzyme and a donor molecule. The enzyme includes a truncated T4 RNAligase 2 lacking a C-terminal segment starting with amino acid residue228 or any higher amino acid residue, and optionally lacking anN-terminal segment starting before amino acid residue 34 or any loweramino acid residue, wherein said enzyme includes a substitution at alocation selected from the group consisting of lysine at position 225,lysine at position 227, arginine at position 55, or a combinationthereof, with a naturally occurring amino acid, and wherein thetruncated T4 RNA ligase is capable of modifying a 3′ hydroxyl group ofRNA. The donor molecule has formula (1):

wherein,

n1=0-25;

R represents H, OH, OCH₃, O(CH₂)₂OCH₃, F, NH₂;

B represents a natural nucleic acid base or base analog, and

when n2=0, R₂ represents H, NH₂, NHQ, —(CH₂)_(n)NH₂, or an aminoalkyllinker having a formula —(CH₂)_(n)NHQ, —O(CH₂)_(n)NH₂, —O(CH₂)_(n)NHQ,wherein n=2 to 18; wherein the alkyl chains represented as (CH₂)_(n) areoptionally substituted with one or more hydroxymethyl groups; andwherein Q represents an active moiety; and

when n2=1, R₂ represents an aminoalkyl linker having a formula—O(CH₂)_(n)NH₂ or —O(CH₂)_(n)NHQ, wherein n=2 to 18; wherein the alkylchains represented as (CH₂)_(n) are optionally substituted with one ormore hydroxymethyl groups; and wherein Q represents an active moiety,

-   -   or formula (2):

wherein,

n1=0-25;

R represents H, OH, OCH₃, O(CH₂)₂OCH₃, F, NH₂;

B represents a natural nucleic acid base or base analog,

X represents —(CH₂)_(n)NH₂, —(CH₂)_(n)NHQ—, —CH═CH—CH—NH₂,—CH═CH—CH—NHQ, —CH═CH—C(═O)—NH—(CH₂)_(n)NH₂,—CH═CH—C(═O)—NH—(CH₂)_(n)NH₂-Q,

-   -   wherein n=2 to 18, or a nucleotide having a pyrimidine base,        said nucleotide carrying an aminolinker at a 5-position of the        pyrimidine base; and wherein Q represents an active moiety.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic showing the mechanism of RNA ligases.

FIGS. 2A and 2B show a gel electrophoresis of reaction products ofmiR-16 (FIG. 2A) miR-21 (FIG. 2B) labeled with Rnl2(1-249)K227Q mutantand AppdCpdC-c₇-NH₂ donor.

FIGS. 3A-3D are graphs showing the time course of labeling reactionswith AppdCpdC-c₇-NH₂ donor by Rnl2(1-249)K227Q for miR-21 (FIG. 3A),Rnl2 (1-249) for miR-21 (FIG. 3B), Rnl2 (1-249) K227Q for miR-16 (FIG.3C) and Rnl2 (1-249) for miR-16 (FIG. 3D).

FIGS. 4A and 4B are graphs showing temperature dependence of theligation reactions in the 22-37° C. range for miR-16 (FIG. 4A) andmiR-21 (FIG. 4B).

FIGS. 5A and 5B show a gel electrophorsis of reaction products for timecourse of labeling reaction by Rnl2(1-249) and Rnl2(1-249)K227Q formiR-21 (FIG. 5B) and miR-16 (FIG. 5A) with AppdCpdC-c₇-NH₂ donor at 0°C. and 10° C.

FIGS. 6A-6H are graphs showing a comparison of the time course oflabeling reactions by Rnl2(1-249) and Rnl2(1-249)K227Q for miR-16 andmiR21 with AppdCpdC-c₇-NH₂ donor at 0° C. and 10° C.

FIG. 7 is a gel showing a ligation reaction with fluorescent labeleddonors using Rnl2(1-249) and Rnl2(1-249)K227Q.

FIG. 8 is a gel showing multiple incorporation of pU using AppU as donorwith the Rnl2(1-249)K227Q mutant.

FIG. 9 shows the structure of donors.

FIG. 10 is a schematic showing the details of the solid phaseadenylation chemistry.

FIGS. 11A and 11B show a reverse phase HPLC (FIG. 11A) and 31P NMR (FIG.11B) characterization of AppdCdC.

FIG. 12 is a gel showing the comparison of commercial ligases andRnl2(1-249)K227Q in the 3′-labeling reaction of miR21 with a 3′-blocked17-nt adenylated adapter oligonucleotide.

DETAILED DESCRIPTION OF THE INVENTION T4 RNA Ligase

The present invention provides a novel truncated T4 RNA (Rnl2) ligaseenzyme. In order for the enzyme to retain its ligating activity, thetruncation should not go beyond amino acid residue 227 at the C-terminalend.

In one embodiment, the truncated Rnl2 ligase lacks the C-terminalsegment starting with amino acid residue 228 or any higher amino acidresidue, i.e, 228-334. In a preferred embodiment, the ligase lacks theC-terminal segment starting with amino acid 235 or any higher residue,i.e., 235-334. In a more preferred embodiment, the ligase lacks theC-terminal segment starting with amino acid 250 or any higher residue,i.e., 250-334. In a most preferred embodiment, the truncated ligaseincludes residues 1-249.

The enzyme also optionally lacks the N-terminal segment before aminoacid residue 34 or any lower amino acid residue. In another embodiment,the enzyme lacks the N-terminal segment before amino acid 5, or anylower amino acid residue.

It is specifically contemplated that, in the truncations describedabove, the phrase “C-terminal segment starting with any designated aminoacid residue (e.g., 228, 235, or 250) or any higher amino acid residue”means that the truncation of the C-terminal segment may start with theamino acid residue designated or with any amino acid residue between thedesignated amino acid residue and the last amino acid residue in fulllength Rnl2 at position 334. For example, in the case where thedesignated amino acid is 228, the truncation may start at amino acidresidue 228, amino acid residue 229, amino acid residue 230, amino acidresidue 231, etc. until amino acid residue 334; in the case where thedesignated amino acid is 235, the truncation may start at amino acidresidue 235, amino acid residue 236, amino acid residue 237, amino acidresidue 238, etc. until amino acid residue 334; and in the case wherethe designated amino acid is 250, the truncation may start at amino acidresidue 250, amino acid residue 251, amino acid residue 252, amino acidresidue 253, etc. until amino acid residue 334.

Similarly, it is specifically contemplated that the phrase “N-terminalsegment starting before any designated amino acid residue (e.g., 34 or5) or any lower amino acid residue” means that the truncation of theN-terminal segment may start at the amino residue immediately precedingthe designated amino acid residue or with any amino acid residue betweenthe amino residue immediately preceding the designated amino acidresidue and the first amino acid residue in full length Rnl2 atposition 1. For example, in the case where the designated amino acid is34, the truncation may start at amino acid residue 33, amino acidresidue 32, amino acid residue 31, amino acid residue 30, etc. untilamino acid residue 1; and in the case where the designated amino acid is4, the truncation may start at amino acid residue 3, amino acid residue2, or amino acid residue 1.

The enzyme can also be mutated. The mutated enzyme includes asubstitution at a location selected from the group consisting of lysineat position 225, lysine at position 227, arginine at position 55, or anycombination thereof, with a naturally occurring amino acid. In apreferred embodiment, the lysine at position 225, lysine at position227, arginine at position 55, or combination thereof is replaced withany of the twenty common naturally occurring amino acid residues thatsignificantly reduce the ability of the enzyme to perform a selfadenylation step, as further described below (FIG. 1, step 2, k−2).

In a preferred embodiment, the enzyme includes at least the substitutionfor the lysine at the 227 position. In this preferred embodiment, thesubstitution at the 227 position can exist by itself, or with either orboth of the substitutions at locations 225 and 55.

Some substitutions are more appropriate than others. For example,conservative replacements of lysine are preferred for the substitutionof lysine at the 225 and/or 227 position. For example, gluatimine,asparagine, threonine and serine are each preferred amino acid residuesfor the substitution because they have similar H-bond interactionpotential as, and a somewhat smaller size than, lysine. Thesesubstitution residues fit in the 225 and/or 227 positions withrelatively little disruption of the enzyme structure. Glutamine is apreferred substitution residue at the 227 position. Other amino acidresidues, for example arginine, are preferably used for substituting thelysine residue at position 225. Lysine is the preferred substitution forthe arginine located at position 55.

Some amino acid residues are less appropriate to use for substitution.For example arginine is a less appropriate candidate and preferably notused for substituting the lysine residue at position 227.

The truncated and mutated enzyme of the invention is capable of ligatinga pre-adenylated donor molecule, as further described below, with the 3′hydroxyl group of an RNA molecule more efficiently and with fewer sidereactions, such as circularization and dimerization, than by meanscurrently used.

The methods of the present invention are useful for modifying any typeof RNA molecule. Some examples of RNA molecules include those thatencode a gene, transfer RNA, messenger RNA, siRNA, and microRNA (miRNA).The RNA may be 5′-phosphorylated or optionally de-phosphorylated.

The enzymes of the present invention may be prepared by methods that arewell known in the art. For example, the enzymes of the invention may bemade synthetically, i.e. from individual amino acids, orsemi-synthetically, i.e. from oligopeptide units or a combination ofoligopeptide units and individual amino acids. Suitable methods forsynthesizing proteins are described by Stuart and Young in “Solid PhasePeptide Synthesis,” Second Edition, Pierce Chemical Company (1984),Solid Phase Peptide Synthesis, Methods Enzymol., 289, Academic Press,Inc, New York (1997).

The enzymes may also be made by isolating or synthesizing DNA encodingthe enzymes, and producing the recombinant protein by expressing theDNA, optionally in a recombinant vector, in a suitable host cell.

Nucleic acids encoding the enzymes of the invention may be synthesizedin vitro. Suitable methods for synthesizing DNA are described byCaruthers et al. 1985. Science 230:281-285 and DNA Structure, Part A:Synthesis and Physical Analysis of DNA, Lilley, D. M. J. and Dahlberg,J. E. (Eds.), Methods Enzymol., 211, Academic Press, Inc., New York(1992).

Nucleic acid molecules encoding the enzymes of the invention may bedesigned or assembled from known nucleic acid sequences encoding wildtype Rnl2 enzymes. An example of a full-length Rnl2 sequence is providedas NCBI Reference Sequence NP_(—)049790, which is incorporated herein byreference, and is provided in SEQ ID NO: 1. Naturally occurring,enzymatically active alleles of this sequence are known. The enzyme ofthe invention, as defined herein, also includes homologues of the enzymehaving an amino acid sequence that differs from SEQ ID NO: 1, but thatpermits the enzyme to retain its ligase activity. For example, acysteine residue (C) may replace the glycine (G) residue that appears atposition 112 of SEQ ID NO. 1. The amino acid difference does not haveknown influences on the function of Rnl2. The sequences of Rnl2 enzymesuseful in the present invention include, for example: (i) SEQ ID NO. 1;(ii) SEQ ID NO. 1 wherein a cysteine residue (C) may replace the glycine(G) residue at position 112; (iii) an enzyme that has Rnl2 ligaseactivity and a sequence that is at least about 95% identical, morepreferably at least about 98% identical, and most preferably at leastabout 99% identical to SEQ ID NO. 1; and (iv) an enzyme that has Rnl2ligase activity and is a naturally occurring allele of SEQ ID NO. 1 witha sequence that is at least about 90% identical, more preferably atleast about 95% identical, most preferably at least about 98% identical,and optimally at least about 99% identical.

As used herein, the term “sequence identity” means nucleic acid or aminoacid sequence identity in two or more aligned sequences, when alignedusing a sequence alignment program. The term “% homology” is usedinterchangeably herein with the term “% identity” herein and refers tothe level of nucleic acid or amino acid sequence identity between two ormore aligned sequences, when aligned using a sequence alignment program.For example, as used herein, 90% homology means the same thing as 90%sequence identity determined by a defined algorithm, and accordingly ahomologue of a given sequence has at least 90% sequence identity over alength of the given sequence.

Optimal alignment of sequences for comparison can be conducted, e.g., bythe local homology algorithm of Smith & Waterman, Adv., Appl. Math.2:482 (1981), by the homology alignment algorithm of Needleman & Wunsch,J. Mol. Biol. 48:443 (1970), by the search for similarity method ofPearson & Lipman, Proc. Nat'l. Acad. Sci. USA 85:2444 (1988), bycomputerized implementations of these algorithms (GAP, BESTFIT, FASTA,and TFASTA in the Wisconsin Genetics Software Package, Genetics ComputerGroup, 575 Science Dr., Madison, Wis.), by the BLAST algorithm, Altschulet al., J. Mol. Biol. 215:403-410 (1990), with software that is publiclyavailable through the National Center for Biotechnology Information, orby visual inspection (see generally, Ausubel et al., infra). Forpurposes of the present invention, optimal alignment of sequences forcomparison is most preferably conducted by the local homology algorithmof Smith & Waterman, Adv. Appl. Math. 2:482 (1981). See, also, Altschul,S. F. et al. 1990 and Altschul, S. F. et al., 1997.

The terms “identical” or percent “identity” in the context of two ormore nucleic acid or protein sequences, refer to two or more sequencesor subsequences that are the same or have a specified percentage ofamino acid residues or nucleotides that are the same, when compared andaligned for maximum correspondences, as measured using one of thesequence comparison algorithms described herein, e.g. the Smith &Waterman algorithm, or by visual inspection.

A plasmid sequence for the expression of Rnl2(1-249) is provided in SEQID NO. 2. Alternatively, the nucleic acid sequence may be derived from aknown Rnl2 amino acid sequence using the genetic code, as is routine tothose of skill in the art.

The preparation of a truncated Rnl2 (amino acid residues 1-249) isdescribed by Ho et al. (2004), which is incorporated herein byreference. In addition, the mutation of the truncated enzyme can beperformed by well known means. For example, the formation, structure andfunction of various Rnl2 mutants are described in Yin et al. (2003),which is incorporated herein by reference. The mutations in thetruncated protein presently claimed can be effected by similar methods.

General methods and procedures for the manipulation of nucleic acids,e.g., polymerase chain reaction (PCR) methods for amplification ofnucleic acids, construction of expression vectors, transformation ofhost cells, and the culture of transformed cells for the production ofprotein are known. These and many more relevant methods may be found ina variety of laboratory manuals, texts and guides. For a general guide,see, for instance, Sambrook & Russel, (2001) Molecular Cloning, Thirdedition, Cold Spring Harbor Press. Other useful sources include: Ausubelet al., 1992 Short Protocols in Molecular Biology, Second edition, JohnWiley & Son; Gene Expression Technology, Methods in Enzymology Vol. 185(ed. David Goeddel et al., Academic Press, Inc., London, 1991); GeneStructure and Expression, Second Edition, J. D. Hawkins (CambridgeUniversity Press, London, 1991); PCR Protocols: A Guide to Methods andApplications (Innis, et al. 1990, Academic Press, San Diego, Calif.);Methods in Molecular Biology (Vol. 7), Gene Transfer and ExpressionProtocols, (ed. E. J. Murray, 1991, The Humana Press Inc., Clifton,N.J.).

A DNA sequence for an expression vector of Rnl2(1-249) K227Q is providedin SEQ. ID. NO 3. In the vector, the protein is fused with an N-terminalHis tag. The tag is comprised of 10 histines. The start codon for theprotein's own first amino acid, Met, starts at position 1074. The ORFends at position 325 (complementary strand). The start codon for thetag, also a Met, starts at position 1140.

Methods of Enzymatically Ligating

The invention further includes methods of enzymatically ligating apre-adenylated donor molecule to the 3′ hydroxyl group of RNA in theabsence of adenosine triphosphate (ATP). In one embodiment, the ligationis conducted in the presence of any of the truncated Rnl2 enzymes thatare substituted at positions 225 and/or 227 and/or 55 (mutated enzymes)as described above. It is an advantage of this embodiment that there isno requirement that the RNA is de-phosphorylated. Therefore, the RNA isoptionally de-phosphorylated at the 5′ location, although it ispreferred that the RNA is not de-phosphorylated. The phosphate group ofthe RNA that is not de-phosphorylated is located at the 5′ position andcan include any 5′ phosphate end, e.g., 5′ phosphate, 5′ di-phosphate,5′ tri-phosphate, etc.

In another embodiment, the ligation is conducted in the presence of anyof the Rnl2 enzymes that are truncated as described above, but that arenot substituted at positions 225 and/or 227 and/or 55. In thisembodiment, the amino acid residues at positions 225 and 227 are bothlysine residues, the residue at position 55 is arginine, and the RNAmust be de-phosphorylated. Also in this embodiment, when both the donormolecule and/or the acceptor molecule are adenylated at the 5′ positionand have a free hydroxyl group at the 3′ position, the molecule hasfewer than sixteen nucleotide residues, preferably fewer than thirteennucleotide residues, more preferably fewer than ten nucleotide residues,most preferably fewer than seven nucleotide residues, and optimallyfewer than four nucleotide residues.

In both of the above embodiments, the pre-adenylated donor molecule canhave formula (1) or formula (2), set forth below.

The donor molecule having formula (2) has both a 5′ adenyl group and a3′ free hydroxyl group. When the 5′ end of the molecule having formula(2) is ligated to the 3′ hydroxyl group of an RNA molecule, the ligatedproduct still has a free 3′ hydroxyl group. Therefore, the resultingligated product can react again with a donor molecule having formula(2). With each addition of a molecule having formula (2), the product ofthe previous reaction increases in length, leading to oligomerization orpolymerization. The increase in length can be controlled by means of thetime the reaction is permitted to proceed.

In the embodiment wherein the ligation is conducted in the presence ofany of the Rnl2 enzymes that are both truncated and mutated as describedabove, n1 in formula (2) represents 0-25 In the embodiment wherein theligation is conducted in the presence of any of the Rnl2 enzymes thatare truncated as described above, but that are not mutated, n1 informula (2) represents 0-15.

In both embodiments, R represents H, OH, OCH₃, O(CH₂)₂OCH₃, F, NH₂ and Brepresents a natural nucleic acid base or base analog in donor moleculeshaving formulas (1) and (2). A natural nucleic acid base is definedherein as any one of the purine or pyrimidine bases commonly found inRNA or DNA, i.e., adenine, guanine, cytosine, thymine or uracil. A baseanalog is any chemical derivative of the nucleic acids, for examplediaminopurine-, which enhances hybridization, or bases with functionalgroup changes, such as 4-thiouridine, for structural studies.

In these donor molecules, when n2=0, R₂ represents H, NH₂, NHQ,—(CH₂)_(n)NH₂, —(CH₂)_(n)NHQ, —O(CH₂)_(n)NH₂, —O(CH₂)_(n)NHQ, whereinn=2 to 18, preferably 2 to 6, and more preferably 3-6, and wherein Qrepresents an active moiety. When n2=1, R₂ represents an aminoalkyllinker having a formula —O(CH₂)_(n)NH₂ or —O(CH₂)_(n)NHQ, wherein n=2 to18, preferably 2 to 6, and more preferably 3 to 6, and wherein Qrepresents an active moiety. The alkyl chains represented as (CH₂)_(n)are optionally substituted with one or more hydroxymethyl groups. Someexamples of R₂ with branched hydroxymethyl substituents include—OCH₂CH—(CH₂OH)(CH₂)₄NH₂ or —CH₂CH—(CH₂OH)(CH₂)₄NHQ.

In formula (2), X represents —(CH₂)_(n)NH₂, —(CH₂)_(n)NHQ—,—CH═CH—CH—NH₂, —CH═CH—CH—NHQ, —CH═CH—C(═O)—NH—(CH₂)_(n)NH₂,—CH═CH—C(═O)—NH—(CH₂)_(n)NH₂-Q, wherein n=2 to 18, or a nucleotidehaving a pyrimidine base, said nucleotide carrying an aminolinker at a5-position of the pyrimidine base; and wherein Q represents an activemoiety.

In a preferred embodiment of the donor molecules of formula (1) andformula (2), n1=0-3, R represents H, B represents cytosine, uridine,thymidine, or adenosine, most preferably cytosine; and R₂ represents—OCH₂CH—(CH₂OH)(CH₂)₄NH₂ or —CH₂CH—(CH₂OH)(CH₂)₄NHQ.

The active moiety Q in formula (1) and formula (2) can be any moietythat performs a useful function, such as assisting in the detection orisolation of the molecule of which the moiety is a part. Examples ofmoieties assisting in detection include fluorescent labels, enzymelabels, radioisotopes, chemiluminescent labels, electrochemiluminescentlabels, and bioluminescent labels, metal particles that can be removedby a magnet, and members of specific molecular binding pairs asdescribed above.

Examples of fluorescent labels which may be used in the inventioninclude 5- or 6-carboxyfluorescein, 6-(fluorescein)-5-(and6)-carboxamido hexanoic acid, fluorescein isothiocyanate, rhodamine,tetramethylrhodamine, and dyes such as Cy2, Cy3, and Cy5, optionallysubstituted coumarin including AMCA, PerCP, phycobiliproteins includingR-phycoerythrin (RPE) and allophycoerythrin (APC), Texas Red, PrincetonRed, green fluorescent protein (GFP) and analogues thereof, andconjugates of R-phycoerythrin or allophycoerythrin, inorganicfluorescent labels such as particles based on semiconductor materiallike coated CdSe nanocrystallites. In a preferred embodiment, the activemoiety is a dye, preferably an organic dye, such as, for example, Cy5,Cy3 or fluorescein.

Examples of enzymatic labels which may be used in the invention includehorse radish peroxidase (HRP), alkaline phosphatase (ALP or AP),beta-galactosidase (GAL), glucose-6-phosphate dehydrogenase,beta-N-acetylglucosamimidase, beta-glucuronidase, invertase, xanthineoxidase, firefly luciferase and glucose oxidase (GO).

Examples of luminescent labels which may be used in the inventioninclude luminol, isoluminol, acridinium esters, 1,2-dioxetanes andpyridopyridazines. Examples of electrochemiluminescent labels includeruthenium derivatives.

Examples of radioactive labels which may be used in the inventioninclude radioactive isotopes of iodide, cobalt, selenium, tritium,carbon, sulfur and phosphorous.

In another preferred embodiment, the active moiety is a member of aspecific molecular binding pair. A specific molecular binding pair asdefined herein is a pair of molecules that specifically bind to eachother. Many different types of specific molecular binding pairs areknown in the art. Some suitable examples include a cellular receptor anda ligand; an antibody and an antigen; and biotin and avidin orstreptavidin. Either member of such pairs are suitable active moietiesfor the purposes of the present invention. Examples of commonly usedspecific molecular binding pairs include biotin/avidin,biotin/streptavidin, and digoxigenin/monoclonal anti-digoxigeninantibody. When an antibody is a member of a specific molecular bindingpair, the whole antibody, or a fragment that includes the binding domainof the antibody, for example, a single chain antibody, may be used.Preferred members of specific molecular binding pairs include biotin anddigoxigenin.

The active molecule can also include lipophylic residues, such ascholesterol. Derivatisation with lipophylic residues is a successfulstrategy to enhance in vivo uptake of small interfering RNAs forpharmaceutical applications.

In another embodiment, the donor molecule has formula (3).

wherein L represents an aminoalkyl linker having a formula—O(CH₂)_(n)NH₂ or —O(CH₂)_(n)NHQ, wherein n=2-18, preferably 2-8, morepreferably 3-6; wherein the alkyl chains represented as (CH₂)_(n) areoptionally substituted with one or more hydroxymethyl groups; andwherein Q represents an active moiety as described above. Donormolecules of formula (3) enable transfer of the L residue as3′-phosphate ester to the 3′ end of RNA with a minimum of structuralchange.

The temperature at which the ligation takes place can be important.Reduction of temperature of the ligation reaction can positivelyinfluence the ligation/circularization ratio. It is preferred that theligation reaction occur at a maximum temperature of about 25° C., morepreferably about 22° C., most preferably about 10° C. Preferably, theligation reaction occurs at a temperature of about 0° C.

The invention also includes DNA molecules encoding any of the truncatedand mutated Rnl2 ligases described above. Further, the inventionincludes kits comprising any of the truncated and mutated Rnl2 ligasesdescribed above as well as any of the donor molecules described above.

Characterization of the Modified RNA Ligase 2 of the Invention thatResults in Favorable RNA 3′ End Ligation

In order to obtain a generally useful, efficient ligase-based labelingor adaptor ligation system, the inventors concluded that a reduction ofthe rate of the self-adenylation step (k⁻²) might provide a solution(FIG. 1). Thus, if the chemical step of E-AMP formation is slow or 0,this will lead to a reduced levels of E-AMP and limit p-miRNAadenylation and following reactions.

FIG. 1 is a schematic showing the mechanism of RNA ligases. The reactionmechanism involves three distinct steps (lower part of the scheme Steps1-3). Step 1 is the reaction with ATP to form a covalent lysyl-N AMPintermediate, termed E-AMP. In step 2 the AMP is transferred to the5′-phosphate end of the donor (pNpNpX) to form an adenylateintermediate, AppNpNpX. In step 3 nucleophilic attack of the acceptor3′-OH forms the new internucleotide linkage. The scheme marks sidereactions from the viewpoint of labeling, i.e. 5′-phosphate activationof 5′ phosphorylated miRNA acceptors (pmiRNA) in Steps 2a and 3a. In theabsence of ATP, the reverse step 2 is a source of E-AMP, leading to sideproducts (circles and dimers) as shown in Step 3a.

The Rnl2(1-249) ligase mutant cannot perform the reverse step 2 reactionof FIG. 1 at low temperature, while still allowing to the intermolecularligation step 3 at only slightly a reduced rate.

The mutant was prepared by exchanging lysine (K) residue 227 byglutamine (Q) and the enzyme is thus referred to as Rnl2(1-249)K227Q.K227 is one of two lysines which interact with the adenylate phosphateduring the AMP phosphoramidate formation on K35, but it is has no directphosphate contact in the rearranged active conformation of the enzyme,which forms immediately before the attack of 3′-OH of the acceptor. K35itself plays multiple roles in this mechanism and cannot be modifiedwithout destroying ligation activity.

When the formation of E-AMP is prevented, intermolecular ligation withadenylated donors occurs essentially without side reactions in a cleantwo-component system. This property of the modified enzyme allows, inprinciple, the solution for the sequence independent 3′ ligationproblem, because residual starting material can be converted to productsby repeated heating-cooling cycles or sufficiently long reaction timesfor miRNAs having inaccessible 3′ hydroxyl groups.

The mutation was performed in the context of the shortened version ofRnl2 lacking the C-terminal domain, i.e. Rnl2(1-249), because theshortened version is much less sensitive to the presence of E-AMP interms of the activation of 5′-monophosphates. The level of requiredreduction of k⁻² by the mutation is therefore expected to be lower withthis enzyme than with the full length version.

The changes caused by the mutation were observed indirectly by measuringthe concentration of the circularization and dimerization byproductsformed in the ligation reaction.

Two model sequences miR-16 and miR-21 were used for comparing theproperties of Rnl2(1-249)K227Q and Rnl2(1-249) in the ligation reactionof 5′-phosphorylated acceptor sequences. FIGS. 2A and 2B show a gelelectrophoresis of reaction products of miR-16 (FIG. 2A) miR-21 (FIG.2B) labeled with Rnl2(1-249)K227Q mutant and AppdCpdC-c₇-NH₂ donor. ThemiRNA was heated to 90° for 30 sec in the ligation buffer, then rapidlycooled to 0° C. The reaction was started by addition of the enzyme at 0°C. Starting concentration of acceptors miR-21 and miR-16 was 50 nM, ofthe donor AppdCpdC-c₇-NH₂ 10 μM. Enzyme concentration was 1.5 μM. RNAligation buffer: 50 mM Tris-HCl, pH 7.5, 10 mM MgCl₂, 10 mMbeta-mercaptoethanol, 0.1 mg/ml acetylated BSA, 15% DMSO. The reactionwas performed at 0° C. Lane 1 contains starting material. Lane 2, 3, 4,5 contain samples at 1, 3, 6, 24 h reaction time. Lane 6 contains sampleat 24 h total incubation time after repeated addition of the enzyme at 6h. The two arrows in FIG. 2B point to the position of dimer and circularside products, which are side products that are much reduced oreliminated in comparison to the use of the non-mutant ligase.

As shown in FIG. 2A, miR-16 has no stabile secondary structuralelements, while FIG. 2B shows miR-21 can form several small hairpinloops. Under optimal conditions at 0° C. the Rnl2(1-249)K227Q mutantperforms the ligation without circularization with these modelsequences. In FIG. 2B, the upper arrow points to the position ofdimerization products and the lower arrow points to the position ofcircularization products.

FIGS. 3A-3D compare the time course of product formation of the two RNAligase enzymes. Reaction conditions were as described for FIG. 2.Symbols: ♦ starting material, ▪ ligation product,  circularizationproduct, ▴ dimer. 32P-labeled compounds were quantified usingautoradiography. The important new property is that for the mutantenzyme no upper limits in product accumulation can be observed. In thecase of the non-mutated enzyme, competitive side reactions set an upperlimit for ligation yield, which is 60% with miR-21, and 80% with miR-16.The origin of the slower ligation rate for the mutant is not understood.Without being bound by theory, the origin may be the chemical step or itmay reflect a slow structural reorganization of a subpopulation ofacceptor sequences, which react on the circularization pathway withRnl2(1-249). Subsequent addition of the enzyme further increased productformation.

FIGS. 4A and 4B are graphs showing temperature dependence of theligation reactions in the 22-37° C. range for miR-16 (FIG. 4A) andmiR-21 (FIG. 4B). The miRNA was heated to 90° for 30 s in the ligationbuffer, then rapidly cooled to 0° C. The reaction was started byaddition of the enzyme at 0° C., followed by bringing the reaction to22° C., 30° C., or 37° C. for 1 hour. Starting concentration ofacceptors miR-16 and miR-21 was 50 nM, of the donor AppdCpdC-c₇-NH₂ 10μM. Enzyme concentration was varied as 1.5, 3.0, or 10.5 μM. RNAligation buffer: 50 mM Tris-HCl, pH 7.5, 10 mM MgCl2, 1 mM DTT, 0.1mg/ml acetylated BSA, 15% DMSO.

As shown in FIGS. 4A and 4B, the optimal reaction temperature forRnl2(1-249)K227Q is between 0 and 10° C. Temperatures above 30° C.inactivate the enzyme. FIGS. 4A and 4B also show that thecircularization and ligation ratio of Rnl2(1-249) is temperaturedependent; ligation is dominant at lower temperatures. The timedependence of ligation product of the mutant follows a similar tendencyas the temperature dependency, while the circularization product ismissing over the whole range. When using longer reaction times (>10 hr)at these higher reaction temperatures some level of circularizationdepending on the sequence of the acceptor, still occurred. With theRnl2(1-249)K227Q, however, no circularization products were detecteduntil 24 hr at 0° C. This is an indication that a specific enzyme donorconformation is required for observing the large reduction in k⁻². Thisaspect of the enzyme is different from the rate reduction of selfadenylation by ATP which is merely 3.8% with the 1(227Q mutation in thefull length Rnl2.

Time course of the reaction for Rnl2(1-249)K227Q and Rnl2(1-249) withmiR-16 and mir-21 is shown in FIGS. 5A and B and 6A-H.

FIGS. 5A and B show a gel electrophorsis of reaction products for timecourse of labeling reaction by Rnl2(1-249) and Rnl2(1-249)K227Q formiR-21 (FIG. 5B) and miR-16 (FIG. 5A) with AppdCpdC-c₇-NH₂ donor at 0°C. and 10° C. Reaction conditions were described for FIG. 2, followed bykeeping the reactions at 0° C., or bringing it to 10° C., with theRnl2(1-249) and Rnl2(1-249)K227Q. Time points were taken at 1, 3, and 17hours. A dilution series of the starting material is presented on theright.

FIGS. 6A-6H are graphs showing a comparison of the time course oflabeling reactions by Rnl2(1-249) and Rnl2(1-249)K227Q for miR-16 (FIG.6A-6D) and miR-21 (FIG. 6E-6H) with AppdCpdC-c₇-NH₂ donor at 0° C. and10° C. Reaction conditions were as described for FIG. 2, followed bykeeping the reactions at 0° C. or bringing it to 10° C., with theRnl2(1-249) and Rnl2(1-249)K227Q. Time points were taken at 1, 3, and 17hours. Symbols: ♦ starting material, ▪ ligation product, circularization product, ▴ dimer. 32P-labeled compounds were quantifiedusing autoradiography.

In FIGS. 5A-5B and 6A-6H, the mutant enzyme shows a slower rate ofproduct formation, depending on acceptor sequence. A hypotheticalexplanation for this effect is that reaction of 3′ end and 5′ end of theacceptors follow different kinetics, so the slow component of the miR21ligation results from a structural reorganization around the 3′ end.With the wild type enzyme this subpopulation undergoes circularizationand the slow component is not observed.

Examples of the structure of possible donor molecules for the ligationreaction are presented in FIG. 9. The AppdCpC-c7-NH2 was synthesized ona commercial aminolinker CPG. Attachment of a fluorescent marker to theterminal amino group does not influence the substrate properties of thedonor.

FIG. 7 is a gel showing a ligation reaction with fluorescent labeleddonors using Rnl2(1-249) and Rnl2(1-249)K227Q. Reaction conditions weredescribed as in FIG. 2. The structure of the fluorescent Cy₅-labeledpre-adenylated compounds is described in FIGS. 6A-H. As shown in FIG. 7,AppdCpC-c7-NH-dye donors are ligated with comparable efficiency toAppdCpC-c7-NH₂. As shown in FIG. 7 lane AppCy5, similar to otherligases, there is no requirement for having a nucleotide residue in theadenylated donor since ADP beta esters react in a comparable way.

FIG. 12 is a gel showing the comparison of commercial ligases withRnl2(1-249)K227Q in the 3′-labeling reaction of miR21 with a 3′-blocked17-nt adenylated adapter oligonucleotide. Experimental conditionsincluded the following: a concentration of donor (App-17-mer-3′-c₇NH₂),1.25 μM acceptor, and 5′-p-miR21 100 nM. All enzymes were used at 1.5 μMconcentration under conditions described in FIG. 2. Reaction time andtemperature were as follows: Rnl1, 37° C., 1 hr; Rnl2, 37° C., 1 hr;Rnl2 (1-249), 22° C., 1 hr; Rnl2 (1-249), 0° C., 1 hr; Rnl2 (1-249)K227Q, 0° C., 1 hr; Rnl2 (1-249) K227Q, 0° C., 16 hr.

In FIG. 12, circularization products were only observed with Rnl-1 andRnl-2. The truncated Rnl2(1-249) gives a mixture of circularization andligation products. The mutated truncated enzyme gives 37% ligationproduct after 1 hr at 0° C., and 80% ligation product after 17 hrincubation.

Chemical Synthesis of Pre-Adenylated Nucleotide Derivatives andOligonucleotides

For this reaction, 5′-adenylated oligonucleotides are required asdonors. An efficient solution for solid phase adenylation reaction ofoligonucleotides was also developed, which complements solid phasesynthetic methods and allows the adenylation reaction to be performedwith comparable efficiency to the usual coupling steps.

Solid phase synthesis of 5′-adenylated oligonucleotides have anadvantage over usual solution methods in that commercially availableamino CPG supports can be used for the synthesis of App-NN—NH₂ typecompounds, which are suitable for post-synthetic labeling with reactivedye derivatives to obtain labeled donors for RNA labeling in the T4 RNAligase catalyzed reaction.

It is difficult to carry out chemical adenylation reactions on the scaleof oligonucleotide synthesis (1-10 μmol) in solution, and purificationof the product mixtures obtained from conventional phosphoanhydridesynthesis methods requires work intensive chromatography orgel-purification techniques.

The synthetic method of the invention is assisted by the oxidativeamidation of oligonucleotide 5′ H-phosphonates. This activation reactioncan be performed very cleanly with trimethylsilylimidazol with supportbound H-phosphonate derivatives, and the resulting 5′phosphorimidazolidate can be converted into the product with excess ofAMP trioctylammonium salt in anhydrous DMF. The oligonucleotideH-phosphonates precursors are accessible on several routes. The use ofsalicylphosphochloridite for the phosphitylation of the 5′ OH ispreferred, because mild hydrolysis conditions are compatible withsuccinate solid supports. The yield of phosphitylation is usually higherthen 90% when the reagent is used in 5-fold excess on 10 μmol scale. Theinventors have found that these conditions are also suitable to workwith standard 1 μmol synthesis columns.

FIG. 10 presents details of the solid phase adenylation with AppU as anexample. The 5′ OH group of support-bound protected oligonucleotides wasphosphitylated with salicylphosphochloridite and the resulting triesterderivatives were hydrolyzed with pyridine-water to give oligonucleotide5′ H-phosphonates in excellent yield. Oxidative amidation of theH-phosphonate was performed with trimethylsilylimidazole intetrachloromethane:acetonitrile (1:1) and the resulting support bound5′-phosphorimidazolidates were reacted immediately with a 30- to 50-foldexcess of AMP trioctylammonium salt in anhydrous DMF. Reaction for 24 hat room temperature resulted in the quantitative formation of thepyrophosphate derivatives as shown by thin layer chromatography of thereaction mixture. After washing the support-bound product with DMF,acetonitrile and water, the product was released from the support andbase protecting groups were removed with concentrated aqueous ammonia.

Ligation of Donors with a Free 3′OH Group

When adenylated donors of the structure AppN are employed, with N as aribonucleoside derivative with a free 3′ OH group, the enzymes describedabove, such as Rnl2(1-249)K227Q, catalyze multiple incorporations of theN residue. The number of N residues incorporated is time dependent.Because the ligase has no specific structure requirement for N, thisproperty of Rnl2(1-249)K227Q will allow the attachment of multiplelabels to the 3′ OH end of any phosphorylated oligoribonucleotide.

The multiple attachment of pU using AppU as a donor with theRnl2(1-249)K227Q mutant is shown in FIG. 8. Lane 1 shows miR-21. Lanes2, 3, 4 show the reaction mixture after 1, 6, 24 h, respectively, underconditions of FIG. 2. Lane 5 shows the results after 24 h with repeatedaddition of the enzyme. The unlabelled lanes represent a time course ofligation of AppdCpdC-c7-NH2 dimer and is not relevant for illustrationof polymerization.

EXAMPLES Example 1 Preparation of the Mutant RNA Ligase 2,Rnl2(1-249)K227Q

The Rnl2(1-249)K227Q mutant was generated using the QuikChange II XL kit(Stratagene). Purification of the mutant protein was performed asdescribed (Ho et al. 2004). Briefly, 11 culture of E. coli Rosetta 2(DE3)/pET16b-Rnl2(1-249)K227Q was grown at 37° C. in Luria-Bertanimedium containing 0.1 mg/ml ampicillin until the A₆₀₀ reached 0.5. Theculture was adjusted to 0.4 mM isopropyl-D-thiogalactopyranoside (IPTG),and incubation was continued at 17° C. for 18 h. Cells were harvested bycentrifugation, and the pellet was stored at −80° C. All subsequentprocedures were performed at 4° C. Thawed bacteria were resuspended in40 ml of buffer A (50 mM Tris-HCl, pH 7.5, 1 M NaCl, 15 mM imidazole,10% sucrose). Lysozyme, PMSF, benzamidine, and Triton X-100 were addedto final concentrations of 1 mg/ml, 0.2 mM, 1 mM, and 0.2%,respectively. The lysates were sonicated to reduce viscosity, andinsoluble material was removed by centrifugation for 40 min at 17,000rpm in a Sorvall SS34 rotor. The soluble extract was mixed with 2 ml ofNi-nitrilotriacetic acid-agarose (Qiagen) for 2 h with constantrotation. The resin was recovered by centrifugation, washed once with 40ml of buffer A three times, and resuspended in 20 ml of buffer A. Theslurry was poured into a column, washed sequentially with 5 ml of bufferA, 5 ml of 50 mM imidazole in buffer B (50 mM Tris-HCl, pH 7.5, 0.2 MNaCl, 10% glycerol), and 5 ml of 100 mM imidazole in buffer BRnl2(1-249)K227Q was step-eluted with 5 ml 200 mM imidazole in buffer B.The polypeptide compositions of the eluate fractions were monitored bySDS-PAGE. The peak fractions were pooled and dialyzed against buffercontaining 50 mM Tris-HCl, pH 8.0, 0.25 M NaCl, 1 mM DTT and 10%glycerol.

Example 2 Synthesis of Adenylated Donors

0.25 M AMP Solution in DMF:

To a stirred solution of AMP free acid (922 mg, 2.5 mmol) in 5 mlmethanol, 1 equivalent tri-n-octylamine (1.09 ml) was added. A clearsolution was obtained within 30 min. The solvent was evaporated to leavethe product as white foam. The residue was coevaporated twice with 10 mlanhydrous DMF. The residue was then dissolved in 10 ml to give a 0.25 Msolution.

Example 3 Procedure for Phosphitylation and Adenylation on 10 μmol Scale

For the synthesis of dimers or trimers reusable Twist columns wereemployed. Oligonucleotide synthesis was performed with 10 μmol CPGsupport loaded with ribonucleosides or with the aminolinker. Aftercompletion of the solid phase synthesis the synthesis column was washedtwice with 5 ml dichloromethane, flushed with argon and dried overnightunder vacuum. Then the CPG was washed with 5 ml dry dioxane-pyridine(3:1) and a syringe filled with 1 ml dioxane-pyridine (3:1) was attachedto the top of the column. An empty syringe was attached to the bottom.Freshly prepared 1 M solution of 2-chloro-4H-1,2,3-dioxaphosphorin-4-onein anhydrous dioxane 50 μl (50 μmol) was injected into the top syringe.The dioxane-pyridine solution was moved between the two syringes for 10min then occasionally for 30 min. The reaction was quenched withpyridine:water (1:1). After 3 h the column was flushed dry with argon,opened and a sample taken to test for the yield of phosphitylation afterammonia treatment. The column containing the 5′-H-phosphonate was washedthree times with 5 ml dry pyridine followed by three washes with 5 ml ofdry acetonitrile:tetrachloromethane (1:1). A syringe filled with 1 mlacetonitrile:tetrachloromethane (1:1) was attached to the top of thecolumn and 330 μmol (44 μl) trimethylsilylimidazole followed by 330 μmol(42 μl) triethylamine was injected into the top syringe. Solvent withreagent was moved up and down for 10 min, after this time approximatelyonce per 5 min. After 30 min, 1 ml acetonitrile:tetrachloromethane (1:1)containing 1 mmol (40 μl methanol) was added with a syringe. After a 5min period another syringe filled with 5 ml 0.25 M AMP trioctylammoniumsalt in DMF (see above) was placed on the top of the column and the DMFsolution was pushed slowly through the column During this step, theheavier acetonitrile:tetrachloromethane (1:1) part is cleanly separatedfrom the DMF and forms a sharp front. The AMP solution was kept incontact with the support for 24 h. The reaction was stopped by washingwith 5 ml DMF to remove the AMP octylammonium salt, three washes of 5 mlmethanol and three more washes of 5 ml water. After flushing the columnwith argon, the product was released from the column by treatment withconcentrated 28% aqueous ammonia for 2 h. Base protecting groups whereremoved by heating for 5 h at 60° C. in a 1.5 ml screw cap tube, andammonia was removed by evaporation on a SpeedVac concentrator.

Product purity was checked by TLC using n-propanol-ammonia (28%)-water(11:7:2) as solvent or reverse phase HPLC. The results for HPLC analysisof the product composition is shown for AppdCpdC in FIG. 11A. Theretention time and product composition by HPLC were pdCpdC 8.63 min(7%), AppdCdC 9.89 min (75%), dCdC 10.35 min (16%).

The structures of the adenylated compounds were verified with ³¹P NMR.FIG. 11B presents an NMR spectrum for AppdCdC reaction mixture afterremoval of protecting groups.

REFERENCES

-   Aravin, A., and Tuschl, T. (2005). Identification and    characterization of small RNAs involved in RNA silencing. FEBS Lett.    579, 5830-5840.-   England, T. E., Gumport, R. I., and Uhlenbeck, O. C. (1977).    Dinucleoside pyrophosphate are substrates for T4-induced RNA ligase.    Proc. Natl. Acad. Sci. USA 74, 4839-4842.-   Ho, C. K. and Shuman, S. 2002. Bacteriophage T4 RNA ligase 2    (gp24.1) exemplifies a family of RNA ligases found in all    phylogenetic domains. Proc Natl Acad Sci USA 99(20): 12709-12714.-   Ho, C. K., Wang, L. K., Lima, C. D., and Shuman, S. (2004).    Structure and mechanism of RNA ligase. Structure (Camb) 12, 327-339.-   Nandakumar, J., Ho, C. K., Lima, C. D., and Shuman, S. 2004. RNA    substrate specificity and structure-guided mutational analysis of    bacteriophage T4 RNA ligase 2. J Biol Chem 279(30): 31337-31347.-   Pfeffer, S., Sewer, A., Lagos-Quintana, M., Sheridan, R., Sander,    C., Zavolan, M., Grasser, F. A., van Dyk, L. F., Ho, C. K., Shuman,    S., et al. (2005). Identification of microRNAs of the herpesvirus    family. Nature Meth. 2, 269-276.-   Pfeffer, S., Zavolan, M., Grasser, F. A., Chien, M., Russo, J. J.,    Ju, J., John, B., Enright, A. J., Marks, D., Sander, C., and    Tuschl, T. (2004). Identification of virus-encoded microRNAs.    Science 304, 734-736.-   Yin S, Ho C K, Shuman S. (2003). Structure-function analysis of T4    RNA ligase 2. J Biol Chem. 278, 17601-17608.

What is claimed is:
 1. A method for enzymatically ligating apre-adenylated donor molecule to RNA, the method comprising reacting thepre-adenylated donor molecule and the 3′ hydroxyl group of a 5′phosphorylated or de-phosphorylated RNA in the absence of adenosinetriphosphate and in the presence of an enzyme comprising a truncated T4RNA ligase 2 (1-249), wherein said enzyme includes a substitution of anarginine with a lysine at a position corresponding to position 55 asencoded by SEQ ID. NO: 3, and wherein the truncated T4 RNA ligase 2 iscapable of ligating the pre-adenylated donor molecule to the 3′ hydroxylgroup of the optionally de-phosphorylated RNA in the absence ofadenosine triphosphate.
 2. The method according to claim 1, wherein saidRNA is dephosphorylated.
 3. The method according to claim 1, whereinsaid RNA is microRNA.
 4. The method according to claim 1, wherein saiddonor molecule has formula:

wherein, n1=0-25; R represents H, OH, OCH₃, O(CH₂)₂OCH₃, F, NH₂; Brepresents a natural nucleic acid base or base analog, and when n2=0, R₂represents H, NH₂, NHQ, —(CH₂)_(n)NH₂, or an aminoalkyl linker having aformula —(CH₂)_(n)NHQ, —O(CH₂)_(n)NH₂, —O(CH₂)_(n)NHQ, wherein n=2 to18; wherein the alkyl chains represented as (CH₂)_(n) are optionallysubstituted with one or more hydroxymethyl groups; and wherein Qrepresents an active moiety; and when n2=1, R₂ represents an aminoalkyllinker having a formula —O(CH₂)_(n)NH₂ or —O(CH₂)_(n)NHQ, wherein n=2 to18; wherein the alkyl chains represented as (CH₂)_(n) are optionallysubstituted with one or more hydroxymethyl groups; and wherein Qrepresents an active moiety.
 5. The method according to claim 4, whereinn=2 to
 6. 6. The method according to claim 4, wherein n1=0 to 3; Rrepresents H; B represents cytosine, uridine, thymidine, or adenosine;and R₂ represents —OCH₂CH—(CH₂OH)(CH₂)₄NH₂ or —CH₂CH—(CH₂OH)(CH₂)₄NHQ.7. The method according to claim 6, wherein B represents cytosine. 8.The method according to claim 4, wherein the active moiety is a dye. 9.The method according to claim 4, wherein the active moiety is an organicdye.
 10. The method according to claim 9, wherein the organic dye isCy5, Cy3 or fluorescein.
 11. The method according to claim 4, whereinthe active moiety is a member of a specific molecular binding pair. 12.The method according to claim 11, wherein the member of a specificmolecular binding pair is biotin or digoxigenin.
 13. The methodaccording to claim 11, wherein the member of a specific molecularbinding pair comprises an antigen.
 14. The method according to claim 11,wherein the member of a specific molecular binding pair comprises abinding domain of a monoclonal antibody.
 15. The method according toclaim 14, wherein the binding domain of a monoclonal antibody is asingle chain antibody.
 16. The method according to claim 4, wherein theactive molecule is cholesterol.
 17. The method according to claim 4wherein said ligating is conducted at a minimum temperature of about 0°C.
 18. The method according to claim 4 wherein said ligating isconducted at a maximum temperature of about 25° C.
 19. The methodaccording to claim 18 wherein the maximum temperature is about 22° C.20. The method according to claim 18 wherein the maximum temperature isabout 10° C.
 21. The method according to claim 1, wherein said donormolecule has formula

wherein, n1=0-25; R represents H, OH, OCH₃, O(CH₂)₂OCH₃, F, NH₂; Brepresents a natural nucleic acid base or base analog, X represents—(CH₂)_(n)NH₂, —(CH₂)_(n)NHQ—, —CH═CH—CH—NH₂, —CH═CH—CH—NHQ,—CH═CH—C(═O)—NH—(CH₂)_(n)NH₂, —CH═CH—C(═O)—NH—(CH₂)_(n)NH₂-Q, whereinn=2 to 18, or a nucleotide having a pyrimidine base, said nucleotidecarrying an aminolinker at a 5-position of the pyrimidine base; andwherein Q represents an active moiety.
 22. The method according to claim21 wherein n1=0-3; R represents H; B represents cytosine, uridine,thymidine, or adenosine; and n=3 to
 6. 23. The method according to claim22, wherein B represents cytosine.
 24. The method according to claim 1,wherein said donor molecule has formula

wherein L represents an aminoalkyl linker having a formula—O(CH₂)_(n)NH₂ or —O(CH₂)_(n)NHQ, wherein n=3 to 6; and Q represents anactive moiety.